1. Technical Field
The present invention relatese to precision phase modulators and, more particularly, to precision phase modulators comprising a plurality of linear suppressed-carrier amplitude modulators in a cascade arrangement.
2. Description of the Prior Art
A desirable component in radio systems is a linear phase or frequency modulator. Linearity is particularly important in multilevel phase modulated systems to enable recovery of transmitted information at the recovery terminal without error.
A linear phase modulator was disclosed in U.S. Pat. No. 4,028,641 issued to W. F. Bodtmann et al on June 7, 1977 which includes a pair of Armstrong modulators in a balanced configuration for eliminating even order nonlinearities. In the Bodtmann et al arrangement, carriers are added which differ in phase from quadrature from the modulating carrier signal by equal and opposite amounts and by properly adjusting the amount of phase difference from quadrature in the added carriers, the third-order nonlinearities can also be substantially eliminated by combining the outputs of the Armstrong modulators in a multiplier circuit.
Another phase modulator circuit is disclosed in U.S. Pat. No. 3,243,730 issued to L. N. Anderson on Mar. 29, 1966 wherein the circuits use cascaded inverters with modulation applied in like phase to all inverters. More particularly, phase modulation is produced in a carrier signal by feeding the carrier signal through an even number of cascaded inverting amplifiers whose individual internal phase shifts are varied by using the modulating signal to vary their bias currents.
The problem remaining in the prior art is to provide a linear phase modulator which permits generation of precisely controlled modulation without sacrificing the desirable modulation and stable properties of the basic Armstrong design.